In a process for manufacturing silicon wafers for the production of semiconductors, when a wafer is cut out from a silicon monocrystalline ingot and processed into a predetermined thickness, etching with sodium hydroxide or potassium hydroxide is performed with the aim of achieving uniform etching. During this process, large amounts of metal impurities contained in the sodium hydroxide or potassium hydroxide are adsorbed on the wafer surface. Generally, these impurities are removed by washing with acids such as dilute hydrofluoric acid. However, particularly in the case of low-resistance substrates, Cu and Ni tend to diffuse within the substrate; among them, because Ni diffuses at approximately 80° C. that is a working temperature of sodium hydroxide solutions, metal impurities diffused inside the substrate cannot be removed by the surface washing with acids, which is a problem.
Moreover, in addition to Cu and Ni, large amounts of transition metals such as Fe are adsorbed on the surface of silicon wafers; therefore, it is necessary to remove them by washing with acidic washing liquids, which in turn makes the entire semiconductor manufacturing process longer and more complex, thereby causing problems of an increase in cost and a decrease in throughput.
Furthermore, in a final process of manufacturing silicon wafers or in a process for manufacturing semiconductor devices, especially with the aim of removing particles, an alkaline washing liquid is used. For example, in a process of making transistors (front end of line), a SC-1 solution which is a mixed solution of ammonia and hydrogen peroxide is frequently used. In a washing process after chemical mechanical polishing (CMP) that constitutes a wiring process, an organic alkali such as tetramethylammonium hydroxide is used. Although these washing liquids do not contain metals as their constituents, metal impurities included in the washing liquids as impurities, or slight amounts of metal impurities introduced from previous processes are still adsorbed on the wafer surface, thereby possibly affecting electrical properties of the wafer.
As described above, because an alkaline washing liquid does not have a cleaning capability for metal impurities, or conversely, it tends to cause metal impurities to adsorb on a substrate surface, the alkaline washing liquid is combined with an acidic washing liquid that can clean metal impurities in a general washing process. Here, the above-mentioned SC-1 solution is used by being combined with a SC-2 solution that is an aqueous solution of hydrochloric acid and hydrogen peroxide, or with dilute hydrofluoric acid. This washing process accounts for approximately ⅓ of the entire semiconductor manufacturing process, and the fact that two kinds of liquids (i.e., alkaline washing liquid and acidic washing liquid) are used in there makes the entire semiconductor manufacturing process longer and more complex, thereby causing problems of an increase in cost and a decrease in throughput.
Furthermore, in manufacturing hard discs which are mass-storage devices, conventionally, contamination by microparticles has been a problem, whereas contamination by metals has not been considered as a problem and washing with alkali and acid has been performed. However, it has been clarified that glass substrates are contaminated by metal impurities contained in alkaline washing liquids and this results in particle contamination; thus, a problem similar to that in silicon substrates occurs.
Therefore, in order to prevent adsorption of metal impurities in an alkaline aqueous solution, the use of various complexing agents (chelating agents) has been proposed. Aminocarboxylic acids such as ethylenediamine tetraacetic acid (EDTA) and diethylenetriamine pentaacetic acid (DTPA) have been known as chelating agents from old times, and their use is proposed also in the field of semiconductor manufacturing (JP A No. 2005-310845 and JP A No. 2006-165408); however, the chelating compounds are unstable and their effects are not sufficient.
In addition, other chelating agents have been proposed, including aminophosphonic acids (JP No. 3503326), condensed phosphoric acids (JP No. 3274834), thiocyanate (JP A No. 2005-038969), nitrite ion and nitrate ion (JP A No. 2005-210085). However, many of these chelating agents are intended to be used in SC-1 solutions; therefore, while they are effective in relatively weak alkaline solutions such as ammonia, they are not effective in strong alkaline solutions such as sodium hydroxide and tetramethylammonium hydroxide.
In recent years, ethylenediamine diortho-hydroxyphenyl acetic acid is also proposed as an effective chelating agent (JP No. 3198878); however, while this chelating agent shows larger effects compared to general chelating agents such as EDTA, it is still unable to prevent adsorption of metals, which is required by manufacturers of semiconductors and silicon wafers.
In addition, there are methods other than the use of chelating agents, including a method to dissolve aluminum (JP A No. 2005-158759), and a method to dissolve silicon or silicon compounds (JP A Hei09-129624); however, since these methods require considerable man power and time to dissolve aluminum or silicon, problems of an increase in cost and a decrease in throughput arise.